Method and apparatus for color measurement of non-solid colors

ABSTRACT

In one embodiment, an apparatus for measuring a color of a non-solid colored sample includes an integrating sphere having a sensor port, a sample port, and a plurality of registration marks affixed to an interior surface of the integrating sphere, outside a periphery of the sample port, a camera positioned near the sensor port, and a plurality of filters positioned between the integrating sphere and camera. An optical axis of the camera extends from the camera, through at least one of the plurality of filters, through the sensor port, to the sample port.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/820,077, filed Aug. 6, 2015, which in turn claims the benefit of U.S.Provisional Patent Application 62/113,684, filed Feb. 9, 2015. Both ofthese applications are herein incorporated by reference in theirentireties.

FIELD OF THE INVENTION

The present invention generally relates to the field of imaging, andmore specifically relates to color measurement.

BACKGROUND

Color measurement systems help to improve operational efficiency andproduct quality in supply chains. For example, color approval officesfor the global apparel supply chain, apparel mills and dye houses, paintstores, textile printing shops, carpet manufacturers, manufacturers ofwood panels, tiles, vinyl sheets, and laminates, and other industriesrelying on the digital color workflow require accurate color evaluationand visualization.

Measurement of non-solid colors (e.g., multiple colors or colorpatterns) in a sample is typically more complicated than the measurementof solid colors. Conventional approaches to measuring non-solid colorsoften cannot produce repeatable results. For example, some approachesrely on a lighting geometry that varies with the position relative tothe sample. Furthermore, such approaches fail to adequately capture thespectral diversity needed to estimate the spectral reflectance of thesample's surfaces.

SUMMARY OF THE INVENTION

In one embodiment, an apparatus for measuring a color of a non-solidcolored sample includes an integrating sphere having a sensor port, asample port, and a plurality of registration marks affixed to aninterior surface of the integrating sphere, outside a periphery of thesample port, a camera positioned near the sensor port, and a pluralityof filters positioned between the integrating sphere and camera. Anoptical axis of the camera extends from the camera, through at least oneof the plurality of filters, through the sensor port, to the sampleport.

In one embodiment, a method for measuring a color of a non-solid coloredsample includes positioning the sample near a sample port of anintegrating sphere, illuminating the sample with diffuse illumination,using the integrating sphere, capturing a plurality of images of thesample through a sensor port of the integrating sphere, subsequent tothe illuminating, wherein each image of the plurality of images iscaptured using a different color filter, wherein at least one of theplurality of images depicts a set of registration marks affixed to aninterior surface of the integrating sphere, and computing a reflectanceof a portion of the sample, based on the plurality of images.

In one embodiment, a computer readable storage device contains anexecutable program for measuring a color of a non-solid colored sample,where the program performs steps including positioning the sample near asample port of an integrating sphere, illuminating the sample withdiffuse illumination, using the integrating sphere, capturing aplurality of images of the sample through a sensor port of theintegrating sphere, subsequent to the illuminating, wherein each imageof the plurality of images is captured using a different color filter,wherein at least one of the plurality of images depicts a set ofregistration marks affixed to an interior surface of the integratingsphere, and computing a reflectance of a portion of the sample, based onthe plurality of images.

In one embodiment, an apparatus for measuring a color of a non-solidcolored sample includes an integrating sphere having a sensor port, asample port, a plurality of registration marks affixed to an interiorsurface of the integrating sphere, outside a periphery of the sampleport, and a light port, a light positioned near the light port, a camerapositioned near the sensor port, and a plurality of filters positionedbetween the integrating sphere and the light source, wherein an opticalaxis of the camera extends from the camera, through the sensor port, tothe sample port.

BRIEF DESCRIPTION OF THE DRAWINGS

The teachings of the present invention can be readily understood byconsidering the following detailed description in conjunction with theaccompanying drawings, in which:

FIG. 1 is a block diagram illustrating one embodiment of a system forcolor measurement, according to the present invention;

FIG. 2 is a schematic diagram illustrating an image of the sample portof the integrating sphere of FIG. 1 in greater detail

FIG. 3 is a flow diagram illustrating one embodiment of a method 300 forcolor measurement of non-solid colors, according to the presentinvention;

FIG. 4 is a high-level block diagram of the present invention that isimplemented using a general purpose computing device; and

FIG. 5 is a block diagram illustrating another embodiment of a systemfor color measurement, according to the present invention.

DETAILED DESCRIPTION

In one embodiment, the present invention is a method and apparatus forcolor measurement of non-solid colors. Embodiments of the invention usean integrating sphere to diffuse light, and then measure the color of anon-solid color sample using a monochrome camera that capturessequential images of the sample through multiple interference filters.The diffuse reflectance of individual pixels of pixel averages can thenbe inferred from the images.

FIG. 1 is a block diagram illustrating one embodiment of a system 100for color measurement, according to the present invention. The system100 is designed to measure the color of a sample, and may be especiallysuited for the measurement of non-solid colors (e.g., multiple colors orcolor patterns) in the sample. The system 100 generally comprises acamera 102, a set of filters 104, and an integrating sphere 106.

In one embodiment, the camera 102 is a monochrome camera. For instance,the camera 102 may be a scientific complementary metal oxidesemiconductor (sCMOS) camera. In one embodiment, the lens configurationof the camera 102 may be optimized to reduce the specular reflectionfrom the other optical components of the system 100.

The set of filters 104 is positioned in front of the lens of the camera102, i.e., along the optical axis O of the camera 102. In an alternativeembodiment, the set of filters 104 is positioned between the lens andthe camera 102. In one embodiment the set of filters 104 includes a setof multiple different interference filters. The set of filters 104 maybe implemented as a filter wheel, for example. In one embodiment, thetransmission spectra of the filters are evenly spaced over substantiallythe entirety of the visible wavelength range (e.g., approximately 400 toapproximately 700 nanometers). In a further embodiment, the transmissionspectrum of each filter is approximately twenty nanometers wide. Thus,in one particular embodiment, the filter wheel includes at least sixteenpossible positions, but may include more or less positions in otherembodiments. In one embodiment, the filter wheel is configured such thatonly one position of the wheel (i.e., only one filter) may be positionedin the optical axis O of the camera 102 at a time.

The integrating sphere 106 is also positioned along the optical axis Oof the camera 102, on the opposite side of the set of filters 104 fromthe camera 102 (i.e., such that the set of filters 104 is positionedbetween the camera 102 and the integrating sphere 106). In oneembodiment, the integrating sphere 106 has a d/8° measurement geometry.The integrated sphere 106 includes a sensor port 112, a sample port 114,and a light port 116. The sensor port 112 and the sample port 114 arealigned with the optical axis O of the camera 102. In one embodiment,the sample port 114 is a large area of view (LAV) port (e.g., a porthaving an area of view of at least approximately thirty millimeters). Inone embodiment, the light port 116 of the integrating sphere 106includes one or more baffles (not shown) positioned to deflectillumination provided by a light source 108.

The light source 108 is positioned near the light port 116 to illuminatethe interior of the integrating sphere 106. In one embodiment, the lightsource 108 is a full-spectrum light source, such as a xenon flash lampor a full-spectrum light emitting diode (LED). For instance, the lightsource 108 may be an LED light source mounted in a luminaire whose frontsurface is flush with a diffuser, which is in turn flush with the lightport 116. The LED itself may be positioned one to two centimetersoutside the integrating sphere 106. Alternatively, the light source 108may be a xenon flash mounted in a cavity that is placed nearly flushwith the light port 116. Or, the light source 108 may be a xenon flashmounted in a cavity and coupled to the light port 116 via a fiber opticlight guide. In any one of these configurations, the integrating sphere106 may include a baffle to prevent direct illumination of the sampleport 114 by the light source 108.

The camera 102, the set of filters 104, and the integrating sphere 106are all coupled to a microprocessor 110 (or other computing device). Themicroprocessor 110 may control operation of the camera 102, the set offilters 104, and the integrating sphere 106.

In the illustrated embodiment, the components of the system 100 arearranged so that that optical axis O of the camera 102—which passes fromthe camera 102, through the filter wheel 104, and through theintegrating sphere 106 to the sample positioned at the sample port114—extends in a substantially parallel orientation (i.e., parallelwithin a few degrees of tolerance) relative to the ground or to asupport surface upon which the system 100 is placed. This horizontalarrangement of components facilitates sample loading and unloading whenthe system 100 is in use.

FIG. 2 is a schematic diagram illustrating an image of the sample port114 of the integrating sphere 106 of FIG. 1 in greater detail. Inparticular, FIG. 2 illustrates an image of the sample port 114 ascaptured through the sensor port 112 of the integrating sphere 106.Thus, the image depicts the sample port 114 as seen from within theintegrating sphere 106. As illustrated, the sample port 114 mainlycomprises an aperture 200 formed in the body of the integrating sphere106 and configured to receive the sample whose color is to be measured.In addition, the sample port 114 includes a set of image registrationmarks 202 ₁-202 ₃ (hereinafter collectively referred to as “imageregistration marks 202”).

In one embodiment, two of the image registration marks 202 arepositioned outside a periphery of the aperture 200, on the interiorsurface of the integrating sphere 106 (i.e., image registration marks202 ₁ and 202 ₂ in FIG. 2). These image registration marks 202 are usedto correct for registration errors due to drift of the system 100between capture of images. For instance, misalignment of the set offilters 104 may cause drift in the position and/or size of the imagescaptured by the camera 102. The two registration marks 202 may be usedto track the size and position of each image. In particular, the twoimage registration marks 202 provide four constraints (i.e., twoconstraints along the x axis, and two constraints along the y axis) andcan correct for up to four registration errors (e.g., displacement alongthe x axis, displacement along the y axis, and image dilation/zoom). Ina further embodiment, a third image registration mark 202 ₃ may bepositioned below aperture 200 and used to correct for rotation drift.

Because the intensity of the light source varies from one acquisition tothe next, a reflecting area that doesn't change from flash-to-flash orfrom one sample to another is needed to compensate for the intensityvariations. Accordingly, a reference channel is adopted as an imagedpart of the integrating sphere outside the sample area. Light from thereference channel 204 is used to compensate for the intensity variationsfrom one flash of light to the next. The reference channel 204 does notreplace the white tile, because: (1) the reference channel 204 is notcalibrated white; and 2) the white tile is acquired on a different flashthan the sample, and so still requires compensation.

The reference channel area 204 is positioned outside a periphery of theaperture 200, and comprises a portion of the interior surface of theintegrating sphere 106. The reference channel area 204 compensates forimage-to-image variation in the intensity of the light source 108. Thereference channel area 204 allows the reference channel to be measuredsimultaneously with the sample. The reference channel area 204 does notnecessarily need to be a calibrated white.

The light intensity distributed over the sample port 114 may benon-uniform. The uneven distribution of illumination can be compensatedfor using images that are captured during conventional white tilecalibration of the system 100. In one embodiment, the white tile is usedas a grey card to calibrate the reflectance measurement at each pixel inan image of the sample port 114.

FIG. 3 is a flow diagram illustrating one embodiment of a method 300 forcolor measurement of non-solid colors, according to the presentinvention. The method 300 may be implemented, for example, by the system100 of FIG. 1 (e.g., under the control of the microprocessor 110). Thus,reference is made in the discussion of the method 300 to variouselements of FIG. 1. It will be appreciated, however, that the method 300could be implemented by a system having a configuration that differsfrom the configuration illustrated in FIG. 1.

The method 300 assumes that certain steps have been performed inadvance, including preparation and loading of the sample whose color isto be measured, and white reference target (e.g., tile) and black trapcalibration of the color measurement system 100. Each of these steps maybe performed in multiple different ways without departing from the scopeof the present invention.

The method 300 begins in step 302.

In step 304, the interior of the integrating sphere 106 is illuminated.In one embodiment, the interior of the integrating sphere 106 isilluminated by the light source 108 positioned near the light port 116of the integrating sphere. The illumination shines into the integratingsphere 106 in such a way that there is no direct path from the lightsource 108 to the sensor port 112 or to the sample port 114. Forinstance, the illumination may be deflected by one or more baffles. Theillumination is thus diffused by multiple reflections from the interiorof the integrating sphere 106 before it is incident on the samplepositioned at the sample port 114. In one embodiment, the sample is anon-solid colored sample, such as a multi-colored or patterned textilesample.

In step 306, the camera 102 captures a plurality of images of thesample. The images are captured through the sensor port 112 of theintegrating sphere 106. In one embodiment, each of the plurality ofimages is captured in a different band of the visible wavelength range(e.g., 400 to 700 nanometers). For instance, each image may be capturedin a different twenty nanometer band of the visible wavelength range(e.g., 400 nm, 420 nm, . . . , 700 nm). In one embodiment, each image iscaptured by placing a different filter in the set of filters 104 infront of the lens of the camera 102.

In step 308, the microprocessor 110 computes the reflectance for atleast one pixel of the images. In one embodiment, the reflectance of apixel is computed by first computing the ratio of the light intensityreflected from the pixel to the light intensity reflected from a whitereference target or tile (e.g., obtained during calibration) for eachcaptured image. Thus, the ratio is computed through each filter in theset of filters 104. In one embodiment, the reference channel is used tocorrect this computation for fluctuations in light intensity that mayoccur between the time of white tile calibration and the time the imagesof the sample are captured.

In one particular embodiment, the sample reflectance, R_(S), may becalculated from a known white reflectance, R_(W) (measured by somereference instrument), and light intensities (various quantities I)measured by the camera 102 as:R _(S)=[R _(W)(I _(s2) −I _(k))(I _(r1) −I _(k))]/[(I _(r2) −I _(k))(I_(w1) −I _(k))]  (EQN. 1)where a subscript of “2” denotes a time of acquisition of a test sample,and a subscript of “1” denotes a time of acquisition of the white tileW. The light intensities, I, are defined as follows:

I_(w1) is the intensity from the white tile, measured at time 1.

I_(s2) is the intensity from the test sample, measured at time 2.

I_(r1) and I_(r2) are the intensities from the reference channelmeasured at times 1 and 2, respectively.

I_(k) is the intensity measured from the black trap, assumed to beindependent of measurement time.

EQN. 1 is expressed entirely in terms of measured light intensities andone known white reflectance (R_(W)). It can be derived from the ratio ofthe following:R _(W) =b(I _(w1) −I _(k))/(I _(r1) −I _(k))  (EQN. 2)R _(S) =b(I _(s2) −I _(k))(I _(r2) −I _(k))  (EQN. 3)Where b is a scale factor that includes two effects: (1) the referencechannel signal is measured at a different location than the sample port;and (2) the reference channel reflectance is not a perfect reflectingdiffuser. Evaluating the ratio of EQN. 3 to EQN. 2 eliminates the factorb.

Referring back to FIG. 3, in step 310, the microprocessor 110 outputsthe reflectance values, e.g., to another computing device or tool.

The method 300 ends in step 312.

FIG. 4 is a high-level block diagram of the present invention that isimplemented using a general purpose computing device 400. The generalpurpose computing device 400 may comprise, for example, themicroprocessor 110 illustrated in FIG. 1. Alternatively, the generalpurpose computing device 400 may be part of another computing devicethat is coupled to the system 100. In one embodiment, a general purposecomputing device 400 comprises a processor 402, a memory 404, ameasurement module 405 and various input/output (I/O) devices 406 suchas a display, a keyboard, a mouse, a stylus, a wireless network accesscard, an image capturing device, and the like. In one embodiment, atleast one I/O device is a storage device (e.g., a disk drive, an opticaldisk drive, a floppy disk drive). It should be understood that themeasurement module 405 can be implemented as a physical device orsubsystem that is coupled to a processor through a communicationchannel.

Alternatively, as discussed above, the measurement module 405 can berepresented by one or more software applications (or even a combinationof software and hardware, e.g., using Application Specific IntegratedCircuits (ASIC)), where the software is loaded from a storage medium(e.g., I/O devices 306) and operated by the processor 402 in the memory404 of the general purpose computing device 300. Thus, in oneembodiment, the measurement module 405 for color measurement ofnon-solid colors as described herein with reference to the precedingFigures, can be stored on a computer readable storage device (e.g., RAM,magnetic or optical drive or diskette, and the like).

It should be noted that although not explicitly specified, one or moresteps of the methods described herein may include a storing, displayingand/or outputting step as required for a particular application. Inother words, any data, records, fields, and/or intermediate resultsdiscussed in the methods can be stored, displayed, and/or outputted toanother device as required for a particular application. Furthermore,steps or blocks in the accompanying Figures that recite a determiningoperation or involve a decision, do not necessarily require that bothbranches of the determining operation be practiced. In other words, oneof the branches of the determining operation can be deemed as anoptional step.

FIG. 5 is a block diagram illustrating another embodiment of a system500 for color measurement, according to the present invention. Thesystem 500 is designed to measure the color of a sample, and, like thesystem 100, may be especially suited for the measurement of non-solidcolors (e.g., multiple colors or color patterns) in the sample. Thesystem 500 generally comprises a camera 502, a set of filters 504, andan integrating sphere 506.

In one embodiment, the camera 502 is a monochrome camera. For instance,the camera 502 may be a scientific complementary metal oxidesemiconductor (sCMOS) camera.

The integrating sphere 506 is positioned along the optical axis O of thecamera 502. In one embodiment, the integrating sphere 506 has a d/8°measurement geometry. The integrated sphere 506 includes a sensor port512, a sample port 514, and a light port 516. The sensor port 512 andthe sample port 514 are aligned with the optical axis O of the camera502. In one embodiment, the sample port 514 is a large area of view(LAV) port (e.g., a port having an area of view of at leastapproximately thirty millimeters). In one embodiment, the light port 516of the integrating sphere 506 includes one or more baffles (not shown)positioned to deflect illumination provided by a light source 508.

The light source 508 is positioned near the light port 516 to illuminatethe interior of the integrating sphere 506. In one embodiment, the lightsource 508 is a full-spectrum light source, such as a xenon flash lampor a full-spectrum light emitting diode (LED).

The set of filters 504 is positioned between the light source 508 andthe integrating sphere 506. In one embodiment the set of filters 504includes a set of multiple different interference filters. The set offilters 504 may be implemented as a filter wheel, for example. In oneembodiment, the transmission spectra of the filters are evenly spacedover substantially the entirety of the visible wavelength range (e.g.,approximately 400 to approximately 700 nanometers). In a furtherembodiment, the transmission spectrum of each filter is approximatelytwenty nanometers wide. Thus, in one particular embodiment, the filterwheel includes at least sixteen possible positions, but may include moreor fewer positions in other embodiments. In one embodiment, the filterwheel is configured such that only one position of the wheel (i.e., onlyone filter) may be positioned at the light port 516 of the integratingsphere 506 at a time.

The camera 502, the set of filters 504, and the integrating sphere 506are all coupled to a microprocessor 510 (or other computing device). Themicroprocessor 510 may control operation of the camera 502, the set offilters 504, and the integrating sphere 506.

Thus, the system 500 is configured in a manner similar to the system100, except that the filters 504 are positioned at the light input(i.e., between the light source 508 and the integrating sphere 506),rather than at the sensor output (i.e., between the camera 102 and theintegrating sphere 106). Positioning the filters 504 to filter the lightsource 508 may limit the amount of light that enters the integratingsphere 506, thereby minimizing the amount by which the integratingsphere 506 heats up. The filters themselves are unlikely to heat updespite their proximity to the light source 508, because they areinterference filters which reflect (i.e., do not absorb) light that isnot transmitted.

The system 500 may be operated in a manner substantially similar to thesystem 100. Thus, the method 300, discussed above, may also beimplemented by the system 500 of FIG. 5 (e.g., under the control of themicroprocessor 510). However, where the plurality of images is capturedin step 306, each image is captured by placing a different filter in theset of filters 104 in front of the light port 516 of the integratingsphere 506, rather than in front of the lens of the camera.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof. Various embodiments presentedherein, or portions thereof, may be combined to create furtherembodiments. Furthermore, terms such as top, side, bottom, front, back,and the like are relative or positional terms and are used with respectto the exemplary embodiments illustrated in the Figures, and as suchthese terms may be interchangeable.

What is claimed is:
 1. An apparatus for measuring a color of a non-solidcolored sample, the apparatus comprising: an integrating spherecomprising: a sensor port; a sample port; and a plurality ofregistration marks affixed to an interior surface of the integratingsphere, outside a periphery of the sample port, wherein the plurality ofregistration marks includes one registration mark positioned below thesample port and used to correct for rotation drift in an image of thesample; a camera positioned near the sensor port; and a plurality ofinterference filters positioned between the integrating sphere andcamera.
 2. The apparatus of claim 1, wherein the camera is a monochromecamera.
 3. The apparatus of claim 1, wherein the plurality ofinterference filters is configured as a filter wheel.
 4. The apparatusof claim 1, wherein transmission spectra of the plurality ofinterference filters are spaced over a range covering all visiblewavelengths of light.
 5. The apparatus of claim 4, wherein the range isapproximately 400 to approximately 700 nanometers.
 6. The apparatus ofclaim 1, further comprising: a light source positioned near a light portof the integrating sphere.
 7. The apparatus of claim 6, wherein thelight source is a full-spectrum light source.
 8. The apparatus of claim6, wherein the light source is a xenon flash lamp.
 9. The apparatus ofclaim 6, wherein the light source is a full-spectrum light emittingdiode.
 10. The apparatus of claim 1, further comprising: a referencechannel comprising a portion of the interior surface of the integratingsphere and positioned outside the periphery of the sample port.
 11. Theapparatus of claim 1, wherein an optical axis of the camera extends in aparallel orientation relative to a support surface upon which theapparatus is positioned and passes from the camera, through at least oneof the plurality of interference filters, through the sensor port, tothe sample port.
 12. A method for measuring a color of a non-solidcolored sample, the method comprising: positioning the sample near asample port of an integrating sphere; illuminating the sample withdiffuse illumination, using the integrating sphere; capturing aplurality of images of the sample through a sensor port of theintegrating sphere, subsequent to the illuminating, wherein each imageof the plurality of images is captured using a different color filter ofa plurality of color filters, and wherein at least one of the pluralityof images depicts a set of registration marks affixed to an interiorsurface of the integrating sphere, wherein the set of registration marksincludes one registration mark positioned below the sample port and usedto correct for rotation drift in the plurality of images; and computinga reflectance of a portion of the sample, based on the plurality ofimages.
 13. The method of claim 12, wherein the capturing is performedusing a monochrome camera, and an optical axis of the monochrome cameraextends in a parallel orientation relative to a support surface uponwhich the integrating sphere is positioned and passes from themonochrome camera, through the color filter, through the sensor port, tothe sample port.
 14. The method of claim 12, wherein at least one of theplurality of images depicts a reference channel comprising a portion ofan interior surface of the integrating sphere.
 15. The method of claim12, wherein the computing comprises: for at least one pixel of theplurality of images, computing a ratio for the at least one pixel, wherethe ratio relates a light intensity reflected from the at least onepixel to a light intensity reflected from a white calibration target.16. A non-transitory computer readable storage device containing anexecutable program for measuring a color of a non-solid colored sample,where the program performs steps comprising: positioning the sample neara sample port of an integrating sphere; illuminating the sample withdiffuse illumination, using the integrating sphere; capturing aplurality of images of the sample through a sensor port of theintegrating sphere, subsequent to the illuminating, wherein each imageof the plurality of images is captured using a different color filter ofa plurality of color filters, and wherein at least one of the pluralityof images depicts a set of registration marks affixed to an interiorsurface of the integrating sphere, wherein the set of registration marksincludes one registration mark positioned below the sample port and usedto correct for rotation drift in the plurality of images; and computinga reflectance of a portion of the sample, based on the plurality ofimages.
 17. The non-transitory computer readable storage device of claim16, wherein at least one of the plurality of images depicts a referencechannel comprising a portion of an interior surface of the integratingsphere.
 18. An apparatus for measuring a color of a non-solid coloredsample, the apparatus comprising: an integrating sphere comprising: asensor port; a sample port; a light port; and a plurality ofregistration marks having non-overlapping centers and affixed to aninterior surface of the integrating sphere, outside a periphery of thesample port, wherein the plurality of registration marks includes oneregistration mark positioned below the sample port and used to correctfor rotation drift an image of the sample; a light positioned near thelight port; a camera positioned near the sensor port; and a plurality ofinterference filters positioned between the integrating sphere and thelight source, wherein an optical axis of the camera extends from thecamera, through the sensor port, to the sample port.
 19. The apparatusof claim 18, further comprising: a reference channel comprising aportion of the interior surface of the integrating sphere and positionedoutside the periphery of the sample port.
 20. The apparatus of claim 18,wherein the plurality of interference filters is configured as a filterwheel.